Not ready to jump into re-cal-ing the e-steps and the requisite recompiling of the firmware. YET. My Kung-Fu is totes weak-sauce.
You don't need to compile the firmware to change the E steps. You CAN set them that way, but you can also just set up a command in your start G code in your slicer like this -
M92 E97.95 ; ***SET NEW E STEP
That will put it in every G code file you make in your slicer automatically.
If you want to set it in the configuration.h file and compile it, you'd set it here -
/**
* Default Axis Steps Per Unit (steps/mm)
* Override with M92
* X, Y, Z, E0 [, E1[, E2...]]
*/
#define DEFAULT_AXIS_STEPS_PER_UNIT { 80, 80, 400,
97.95 }
and you calculate the new value for these settings like this -
100 mm
-------------- * current E steps = new E steps
actual mm
(See tutorials on YT)